The present invention relates to an oil-impregnated capacitor containing a plastic film or paper as a dielectric film.
In oil-impregnated capacitors, it is a very important and valuable means for realizing a small and light oil-impregnated capacitor that to increase the voltage per unit thickness applied to a dielectric layer. However, even if the potential stress is merely designed, there is such a problem that the resulting oil-impregnated capacitor has an enlarged dissipation factor, though the degree of the enlargement varies a little depending on the kinds of its dielectric film or paper and dielectric liquid, so that the thermal deterioration and the thermal destruction of the oil-impregnated capacitor are accelerated, resulting in impossibility to put the oil-impregnated capacitor to practical use as a capacitor. In particular, this tendency is more marked in capacitors used at higher temperatures, and is a serious problem in rendering oil-impregnated capacitors used at high temperatures smaller and lighter.
In order to improve these severe subjects, it has heretofore been proposed to incorporate so-called scavenger materials or getter materials into a liquid impregnant, but the demand for design of a high potential stress is still strong and now the subjects cannot be improved yet by addition of conventional scavenger materials. These conventional scavengers are used for capturing various impurities present in an insulating oil and various decomposition products produced during performance to establish the long-term stability of the capacitors. That is to say, their objects are as follows. The liquid impregnant is decomposed by thermal and electric energies during the performance and undergoes additional reaction with the decomposition products produced in the decomposition to deteriorate the electric characteristics (insulating characteristics, dielectric characteristics, etc.) of the liquid impregnant, and therefore this trouble is prevented by adding the scavengers. Dielectric films, for example, polyolefin films are contaminated with slight amounts of a metal chloride and chlorine because the metal chloride is used as a catalyst in the production process of the films, and the contaminants get mixed in the liquid impregnant to bring about disadvantageous effects on the electric characteristics of the liquid impregnant, and therefore this trouble is prevented by adding the scavengers.
As to the scavenger materials, epoxide compounds which does not contain metallic atom within their molecular structures are known to be particularly useful. For example, it has been proposed in U.S. Pat. No. 3,242,401 (Katchman) to incorporate dipentene dioxide as a scavenger into a liquid impregnant comprising a chlorinated aromatic compound. In U.S. Pat. No. 3,754,173 (Eustance), it has been proposed in order to stabilize the capacitors to incorporate epoxides into a liquid aromatic ester, and to neutralize impurities such as chlorine compounds having got mixed with the liquid impregnant from a dielectric film and decomposition products during the performance.
However, the liquid impregnants containing scavenger materials prosess inferior dielectric characteristics, in particular, an enlarged dissipation factor. Therefore, for example, when an oil-impregnated capacitor is used under a higher voltage stress condition, the amounts of the impurities produced by the decomposition during the performance are increased, therefore, even if a large amount of a scavenger is incorporated in order to increase the probability of neutralization of large amounts of the decomposition impurites, there is caused, contrary to the purpose, enlargement of dissipation factor and reduction of insulating resistance of the resulting oil-impregnated capacitor, so that the long-term stability of the capacitor is lowered, resulting in impossibility to design a high potential stress. Particularly when the oil-impregnated capacitor is used at a high temperature and at a higher voltage stress, the enlargement of the initial dissipation factor results in increase of the quantity of heat generated by the oil-impregnated capacitor, great case of thermal breakdown and reduction of the service life of the oil-impregnated capacitor.
Accordingly, in the case of oil-impregnated capacitors containing a stabilized liquid impregnant prepared by incorporating conventional scavenger or getter materials represented by epoxide compounds, the design of rated voltage (i.e. voltage stress) and temperature is limited, and it has been very difficult to obtain an oil-impregnated capacitor which can be used under higher potential and thermal stresses.
In short, the dissipation factor of the oil-impregnated capacitor used under a higher voltage stress condition becomes larger as compared with that of the capacitor used under a low voltage stress condition, so that much generation of heat accelerates thermal breakdown, and at last the capacitor is led to short service life, though the degree of these troubles vary depending on the kind of the liquid impregnant. There have heretofore been used stabilized impregnants improved in the above-mentioned problems by neutralizing impurities present in the liquid impregnant and decomposition products produced during performance by adding scavenger or getter materials represented by epoxide compounds, but they have been insufficient for use under higher stress conditions, in particular, at a high temperature and under a high voltage stress. That is to say, this is because under conditions of a high temperature and a high voltage stress, the amounts of the decomposition products produced during performance are increased, and increasing the amount of the scavenger material in order to neutralize large amounts of these impurities causes an enlargement of the initial dissipation factor and a lowering of the insulation resistance, so that the resulting capacitor has only a shorter service life. Further, as to the impurities which deteriorate the characteristics of oil-impregnated capacitors and/or shorter the life of the capacitors, there exist not only organic impurities but also inorganic impurities, metal ions and the like. In particular, oil-impregnated capacitors having vapor-deposited metal layers as a pair of electrodes are advantageous in that the weak portion of its dielectric layer is cleaned by self-heating action, but on the other side, a large amount of metal ions are produced. For removing or neutralizing these impurities, conventional scavenger materials such as epoxide compounds and the like have been insufficient.
Here, it has been found that incorporation of a liquid impregnant with organometallic compounds, for example, organosilane compounds or organotitanate compounds greatly improves the dissipation factor of an oil-impregnated capacitor having a plastic film and/or paper as a dielectric film, and makes it possible to prolong the service life of the oil-pregnated capacitor under heterofore unusable higher stress conditions, in particular, under the circumstances of a high temperature and a high voltage.